1. Field of the Invention
The present invention generally relates to storage devices, and more particularly to servo systems for compensating for lateral tape motion.
2. Related Art
Increased data storage capacity and retrieval performance are desired of all commercially viable mass storage devices and media. In the case of linear tape recording, a popular trend is toward multi-head, multi-channel head structures with narrowed recording gaps and data track widths, so that many linear data tracks may be laid on a tape medium of a predetermined width, such as one-half inch width tape. To increase the storage density for a given cartridge size, the bits on the tape may be written to smaller areas and on a plurality of parallel longitudinal tracks. As more data tracks are recorded on a tape, each track becomes increasingly narrow. The tape therefore becomes more susceptible to errors caused from the tape shifting up or down (called lateral tape motion or “LTM”) in a direction perpendicular to the tape travel path as the tape passes by the magnetic head. LTM may be caused by many factors including tape slitting variations, tension variations, imperfections in the guiding mechanism, friction variations, and environmental factors such as heat and humidity. These factors affect LTM in various ways. Some may cause abrupt momentary jumps while others may cause a static shift. Generally, LTM is unpredictable and unrepeatable.
In multi-head, multi-channel magnetic tape storage systems, random lateral tape motion is generally a limiting factor in achieving higher track densities, and thus higher user data capacity. In order to maintain proper alignment of the head with the storage tape and data tracks on the tape, the tape is generally mechanically constrained to minimize LTM and data retrieval errors. Mis-registration between the head and data tracks can cause data errors during readback, and data loss on adjacent tracks during writing.
Various techniques for increasing the track density on magnetic tape employ recording and maintaining servo information on the tape to provide positioning information to a tape drive system during writing and/or reading processes. Some systems magnetically prerecord a continuous track of servo information which is then read and used as a position reference signal. For example, a variety of techniques have been used including dedicated, embedded magnetic servo tracks, time and amplitude magnetic servo tracks, and the like. One disadvantage of prewritten servo tracks is that the use of a separate servo track writer introduces errors into the system, reducing the ability to obtain the smallest track width.